The present invention relates to a process for electrochemical roughening of aluminum for printing plate supports.
DE-A 3,717,654 discloses a process for electrochemical roughening of aluminum or aluminum alloys for printing plate supports by means of utilizing alternating current in an acidic electrolyte which contains sulfate ions and chloride ions, wherein the chloride ions are present in the form of aluminum chloride. Very uniform, scar-free support surfaces with fine roughening are obtained, which have excellent lithographic properties, but, precisely because of the fine roughening, the anchorage of the ink-bearing organic layer on the support is unsatisfactory. This leads to a shorter print run compared to a printing form in which a support is used which is produced by a process utilizing electrolytes which are free of sulfate ions but contain chloride ions or nitrate ions.
Printing plates, particularly offset printing plates, are comprised of a support and at least one radiation-sensitive layer located thereon, this layer being applied to the layer support by the customer in the case of non-precoated plates or by the industrial manufacturer in the case of precoated plates.
Aluminum or aluminum alloy has gained acceptance as a layer support in the printing plate field. These layer supports can, in principle, be used without a pretreatment, but they are, in general, treated in or on the surface, for example by mechanical, chemical and/or electrochemical roughening, a chemical or electrochemical oxidation and/or a treatment with agents conferring hydrophilic character. Chemical and electrochemical roughening is also referred to as "graining" or "etching".
In the modern, continuously operating high-speed installations of manufacturers of printing plate supports and/or precoated printing plates, a combination of the above-mentioned treatments frequently is employed, in particular, a combination of electrochemical roughening and anodic oxidation, if appropriate with a subsequent stage conferring hydrophilic character.
The roughening can be carried out in aqueous acids such as aqueous HCl or HNO.sub.3 solutions or in aqueous salt solutions such as aqueous NaCl or Al(NO.sub.3).sub.3 solutions, using alternating current. The peak-to-valley heights of the roughened surface, thus obtainable, expressed as mean peak-to-valley heights Rz, are in the range from 1 to 15 .mu.m, especially in the range from 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768 (October 1970). As the mean peak-to-valley height Rz, the arithmetic mean is calculated from the individual peak-to-valley heights of five adjacent individual measuring sections.
The roughening is carried out, inter alia, for improving the adhesion of the reproduction layer to the layer support and the damping water holding of the printing form produced from the printing plate by exposure and development.
Water holding is an important quality feature for offset printing plates. In the publication "Ermittlung einer optimalen Wasserfuhrung zur Steigerung der Leistungsfahigkeit des Offsetdruckes [Determination of Optimum Water Holding for Improving the Performance of Offset Printing]" (J. Albrecht; W. Rebner and B. Wirz, Westdeutscher Verlag, Koln and Opladen, 1966, page 7), water holding is defined as the dosage and control of the damping of the printing form during the print run. The water holding depends, inter alia, on the surface roughness of the printing form, i.e., the graining of the surface. The problems of insufficient water holding are well-known. If too much water is required in order to keep the non-printing areas of a printing form free of ink, additional water can be emulsified into the ink, and the print becomes flat. Moreover, water marks can arise if the paper becomes moist. Furthermore, register problems can arise, and, in web-offset printing, there is an increased risk of the paper web tearing. Only some of the problems associated with water holding are mentioned here. Reference to the importance of proper water holding is also made in the publication "Beitrag zur Analyse des Offsetprozesses [Contribution to the Analysis of the Offset Process]", (P. Decker; Polygraph Verlag, Frankfurt am Main pages 17 and 18). In this publication, the consequences of too high and too low damping water holding are discussed. The term "damping water holding" is more appropriate than the term "water holding" because pure water is generally not used in offset printing for damping since several components typically are added to the water.
The disadvantages, already mentioned above, of excessive damping water are listed in the cited publications. An insufficient amount of damping water is also a disadvantage. If the printing plate is provided in the printing press with insufficient damping water because of too low a setting of the damping unit, or, if the printing plate requires more damping water than the damping unit of the printing press can supply due to structural limitations or other reasons, non-printing areas of the printing plate can also absorb ink and participate in printing, fine half-tone areas being particularly sensitive to participation in printing. The participation of non-image areas in printing within half-tone areas is known as "smearing."
What is desirable is thus a printing plate which requires only a small amount of damping water for keeping fine half-tones and large non-image areas free of ink, and, also demonstrates neutral behavior toward large quantities of damping water and still give excellent prints even if the damping water available temporarily exceeds the normal quantity due to fluctuations inherent in operation.
The damping water consumption of a printing plate can be measured objectively with sufficient accuracy, but not the damping water holding, since no objective measurement method exists for some of the above-mentioned disadvantageous phenomena such as, for example, smearing (P. Decker, in "Beitrag zur Analyse . . . [Contribution to the Analysis . . . ]", page 18). Therefore, the damping water holding of a printing plate herein is assessed qualitatively by the relative terms "very good", "good", "satisfactory", "adequate", "moderate", "poor" and "very poor."
Due to the exposure or irradiation and developing, or decoating in the case of electrophotographically operating reproduction layers, the image areas, which are ink-bearing during the later printing, and the damping water-bearing non-image areas, which in general represent the exposed support surface, are produced on the printing plate, whereby the actual printing form results. Widely different parameters affect the later topography and hence the damping water holding of the surface to be roughened. Information on this subject is provided, for example, in the literature references listed below.
In the article "The Alternating Current Etching of Aluminum Lithographic Sheet" by A. J. Dowell in (Transactions of the Institute of Metal Finishing), 1979, Volume 57, pages 138 to 144, the effects of varying the process parameters in the roughening of aluminum in aqueous hydrochloric acid solutions are investigated and discussed. The electrolyte composition is changed with repeated use of the electrolyte, for example, with respect to the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable via the pH) and the Al.sup.3+ ion concentration, and effects on the surface topography are observed. Varying the temperature variation between 16.degree. C. and 90.degree. C. effects the roughening only at about 50.degree. C. and above, which manifests itself, for example, by a sharp decrease in layer formation on the surface. Utilization of a roughening time between 2 and 25 minutes leads, with increasing time of action, to increasing dissolution of metal. Varying the current density between 2 and 8 A/dm.sup.2 results in higher roughness values with increasing current density. If the acid concentration is varied in the range from 0.17 to 3.3% of HCl, only insignificant changes in the hole structure arise between 0.5 and 2% of HCl, only local attack on the surface takes place below 0.5% of HCl, and irregular dissolution of aluminum takes place at high values. If pulsed direct current is used instead of alternating current, it is found that evidently both half-wave types are necessary for uniform roughening. Moreover, the article points out that the addition of sulfate ions increasingly leads to undesired, coarse, nonhomogeneously roughened structures which are unsuitable for lithographic purposes.
The use of hydrochloric acid for roughening substrates of aluminum is known. Uniform graining, which is suitable for lithographic plates and is within a useful roughness range, can be obtained in this way. A difficulty with pure hydrochloric acid electrolytes is adjusting the operating conditions to obtain a flat and uniform surface topography, and thus it is necessary to adhere to operating conditions within very narrow limits.
The influence of the composition of the electrolyte on the roughening quality is also described, for example, in the following publications:
United Kingdom Patent No. 1,400,918 mentions aqueous solutions having a content from 1.2 to 1.5% by weight of HNO.sub.3 or from 0.4 to 0.6% by weight of HCl and, if appropriate, 0.4 to 0.6% by weight of H.sub.3 PO.sub.4 as the electrolyte in the alternating current roughening of aluminum for printing plate supports, and
U.S. Pat. No. 4,072,589 mentions aqueous solutions having a content from 0.2 to 1.0% by weight of HCl and 0.8 to 6.0% by weight of HNO.sub.3 as the electrolyte in the alternating current roughening of aluminum.
Additives to the HCl electrolyte have the objective of preventing a disadvantageous, local attack in the form of deep holes. Thus,
U.S. Pat. No. 4,172,772 describes the addition of monocarboxylic acids such as acetic acid to hydrochloric acid electrolytes,
U.S. Pat. No. 3,963,594 describes the addition of gluconic acid,
EP-A-0,036,672 describes the addition of citric acid and malonic acid, and
U.S. Pat. No. 4,052,275 describes the addition of tartaric acid.
All these organic electrolyte constituents have the disadvantage that, at high current load which is to be equated to high voltage load, they are electrochemically unstable and decompose.
In DE-A 3,503,927, ammonium chloride is described as an inorganic additive to an HCl electrolyte.
Inhibiting additives, such as phosphoric acid or chromic acid as described in U.S. Pat. No. 3,887,447, and boric acid as described in U.S. Pat. No. 3,980,539, have the disadvantage that the protective action frequently collapses locally and individual, particularly pronounced scars correspondingly can form there.
Japanese Application 91,334/78 has disclosed alternating current roughening in an electrolyte of hydrochloric acid and an alkali metal halide to produce a lithographic support material.
In U.S. Pat. Nos. 3,632,486 and No. 3,766,043, direct current roughening in dilute hydrofluoric acid is mentioned, the Al strip being connected as the cathode.
Another known possibility for improving the roughening uniformity is modifying the type of current used, which includes, for example,
alternating current, wherein the anode voltage and the anodic Coulomb input are greater than the cathode voltage and the cathodic Coulomb input according to U.S. Pat. No. 4,087,341, the anodic half period of the alternating current being in general adjusted to be less than the cathodic half period; this method is also referred to, for example, in U.S. Pat. Nos. 4,301,229 and No. 4,272,342 and United Kingdom Patent No. 2,047,274
alternating current, wherein the anode voltage is markedly increased as compared with the cathode voltage according to U.S. Pat. No. 3,193,485 and
interruption of the alternating current flow for 10 to 120 seconds or 30 to 300 seconds, wherein the electrolyte is an aqueous 0.75 to 2 N HCl solution which includes a NaCl or MgCl.sub.2 additive, according to United Kingdom Patent No. 879,768. A similar process with an interruption of the current flow in the anode phase or cathode phase is also described in U.S. Pat. No. 4,294,672.
Though the above-discussed methods provide relatively uniformly roughened aluminum surfaces, they require relatively very expensive equipment and are operable only within very narrow parameter limits.
Another known procedure is the combination of two roughening processes. This has the advantage over a single-stage process in that, depending on the process method, the influence of one or the other stage can predominate within certain limits predetermined by the properties of the individual stages.
According to the methods described in U.S. Pat. No. 3,929,591; United Kingdom Patent No. 1,582,620; JP-A 123,204/78; United Kingdom Patents No. 2,058,136 and No. 2,060,923; EP-A 0,131,926; United Kingdom Patent No. 2,047,274 and JP-B 16,918/82, the combination of prestructuring occurs mechanically in the first step, followed by chemical cleaning (pickling), which may be carried out with electrochemical roughening by means of modified alternating current in electrolytes containing hydrochloric acid or nitric acid, it being possible for a further cleaning step then to take place.
These processes exploit the advantage of double roughening, with mechanical roughening as the first step, whereby especially a saving in current is achieved.
For the manufacture of capacitors from aluminum foils, various two-stage processes are known. In U.S. Pat. No. 4,525,249, a process is described which uses hydrochloric acid in the first stage and in which the aluminum foil, in the second stage, is treated currentlessly with a dilute nitric acid which additionally contains aluminum in the form of aluminum nitrate. This process does not give surfaces which can satisfy the stringent requirements presently demanded for offset printing plates.
Two-stage processes which use electrochemical methods in both stages have also been disclosed. In the process according to U.S. Pat. No. 4,721,552, the first electrolyte contains hydrochloric acid, whereas the second electrolyte can also contain hydrochloric acid in addition to nitric acid. A similar process is described in Japanese Publication JP 61 051,396. Although these known processes give surfaces useful for lithographic purposes, the fineness of their surface structure does not reach that which is obtained according to the teaching of German Offenlegungsschrift No. 3,717,654.
U.S. Pat. No. 4,437,955 discloses a two-stage electrochemical roughening process for the manufacture of capacitors, employing an electrolyte containing hydrochloric acid in the first step and an electrolyte containing chloride ions and sulfate ions in the second step. The electrolyte of the second stage is not acidic, and direct current is used in this stage.
A further two-stage electrochemical process for manufacturing a capacitor foil is described in U.S. Pat. No. 4,518,471. The electrolytes in both baths are identical and contain dilute hydrochloric acid and aluminum ions. The baths are operated at different temperatures, namely, at 70.degree. to 85.degree. C. in the first stage and at 75.degree. to 90.degree. C. in the second stage.
The surfaces produced in the two last-mentioned processes, optimized for electrolyte capacitors, are too scarred for application in lithography.